The background of the invention will be discussed in two parts:
1. Field of the Invention
This invention relates to aluminum melting furnaces, and more particularly to an aluminum melting furnace using radiation as the primary source of heating in the aluminum bath.
2. Description of the Prior Art
In the melting of metals in a furnace, such as in the melting of aluminum, prior art furnaces have utilized electrical resistance as a source of heat as well as petroleum or gas products. With electrical resistance heating, there are no combustion by-products, thus giving a clean melt, that is there are no impurities imparted to the bath. One such resistance heat furnace is manufactured and sold in the United States by Ajax Magnathermic Corp., of Warren, Ohio. The particular furnace is referred to as a RAM resistance furnace, and utilizes overhead resistance elements in the form of rods of silicon carbide mounted in a removable roof, the rods being connected in parallel to a source of electrical energy. The aluminum bath lies immediately below the resistance elements. However, the use of electricity as a means of heating aluminum is cost prohibitive in regions having abundant natural gas supplies for energy while lacking cheap electrical power.
A typical prior art furnace apparatus is shown and described in U.S. Pat. No. 2,264,740, issued to Brown on Dec. 2, 1941 for a Melting and Holding Furnace. In this patent, the combustion and melting chamber are one and the same with the flame from the burner directly impacting on the surface of the metal. In such prior art arrangements, dross is formed on the surface due to the reaction between the flame and the products of combustion with the metal, thus reducing the effective yield of metal.
Subsequently, melting furnaces employed arches of heat-conducting refractory such as silicon carbide, as shown in the furnace apparatus of U.S. Pat. No. 2,298,055 and U.S. Pat. No. 2,331,992, the first being issued to Hulme et al on Oct. 6, 1942 and the second being issued to MacDonell on Oct. 19, 1943. In these furnaces, referred to as muffle furnaces, the arch separates the combustion area from the melting chamber, and both employ a forward flame burner apparatus. In such furnaces, although the flame does not come in direct contact with the surface of the molten bath, the configuration results in hot spots and cold spots along the surface of the arch, thereby resulting in uneven heating of the metal. Furthermore, with a large flue opening, much of the heat escapes with the products of combustion.
In U.S. Pat. No. 3,633,665, issued to Sparks, on Jan. 11, 1972, silicon carbide is used as a floating heat conductor, in block form, for forestalling the formation of dross on the surface and for passing heat into the molten bath.
Other attempts to utilize the properties of silicon carbide in a furnace apparatus are shown and described in U.S. Pat. Nos. 3,724,447 and 3,635,457 issued to Parkhill et al on Apr. 3, 1973 and to King on Jan. 18, 1972, respectively. In both of these patents a crucible or housing of silicon carbide is immersed in the bath with the heat or flame being directed into the vessel for transfer of the heat to the bath by this contact. In the King Patent, the crucible has an open top with the result being that the products of combustion are directed over the surface of the molten bath, thus permitting the formation of oxides. With the intrusion of the crucible into the melting chamber, the volume of the melting chamber is reduced, thus requiring a larger furnace for a given volume of melt, with a correspondingly higher cost of construction. Correspondingly, with the smaller housing of Parkhill et al, for efficient heat transfer through the walls of the housing, the chamber size must be smaller. In either furnace, the crucible or housing is subjected to physical and thermal, as well as chemical abuse from the direct contact with the solid or molten metal thus requiring replacement and consequent higher operating costs and downtime.
The U.S. Pat. No. 2,385,333 issued Sept. 25, 1945 to Clapp et all illustrates an attempt to utilize the properties of silicon carbide while using the combustion gases as an additional heating source. In this furnace, a blanket in the form of a silicon carbide or similar composition tile is used. As shown, there are a series of identically configured generally rectangular dish-shaped tiles adapted to rest on the surface of the melt, or to rest on ledges, depending on the level of the bath. In addition, to provide use of the flue gas as a heat source, the flue gases circulate through ducts surrounding the melting chamber. A muffle arch is also employed as part of the apparatus, with gases from the arch passing into the ducts.
Another furnace utilizing ducts in thermal contact with the melting chamber is shown and described in U.S. Pat. No. 2,331,887 issued Oct. 19, 1943 to Bonsack. In the Bonsack furnace, forward flame burners are used with the flame being passed into tubular silicon carbide members disposed in the upper portion of the furnace interior. The flue gases communicate directly with the elaborate series of ducts or passageways encircling the melting chamber to provide radiated heat through the chamber walls.
While both of the furnaces of Bonsack and Clapp preclude direct contact between the flame and the by-products thereof, with the surface of the bath, both constructions are unduly complex, inordinately expensive to construct and subject to extensive work being required for overhauling the furnace due to the ductwork.
Accordingly, it is an object of the present invention to provide a new and improved furnace construction using a silicon carbide member as a primary source of radiant heat for melting the metal.
It is another object of the present invention to provide a new and improved furnace apparatus using a flat flame burner in conjunction with a silicon carbide plate member as a uniformly heated radiation member for melting aluminum or the like.
It is still another object of the present invention to provide a new and improved clamshell furnace having a hinged top cover utilizing ceramic fibers for insulation.
It is yet another object of the present invention to provide a new and improved, compact thermally efficient clamshell furnace for melting aluminum or the like.